1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor and an electric power steering apparatus, and more particularly to a magnetostrictive torque sensor having a suitable structure for reducing a variation in a sensor sensitivity characteristic caused by heating a rotating shaft at a step of adding a magnetic anisotropy, and an electric power steering apparatus mounting the magnetostrictive torque sensor thereon.
2. Related Art
For example, in an electric power steering apparatus provided as a steering system of a vehicle, a steering torque added from a steering wheel to a steering shaft by a steering operation of a driver is generally detected by a steering torque detecting portion. The steering torque detecting portion is usually constituted by a torsion bar type torque sensor. Recently, a magnetostrictive torque sensor has also been proposed. The steering shaft functions as a rotating shaft to be rotated upon receipt of a rotating force by a steering operation and serves as a rotating shaft in the steering torque detecting portion. The electric power steering apparatus controls a driving operation of a steering force aiding motor in response to a torque signal detected through the steering torque detecting portion and relieves a steering force of a driver to give a comfortable steering feeling.
As described above, the magnetostrictive torque sensor has been known as a steering torque detecting portion to be used for the electric power steering apparatus. The magnetostrictive torque sensor is provided with a magnetostrictive film having magnetic anisotropies in reverse directions to each other in two predetermined places of a surface of the steering shaft. The magnetostrictive torque sensor has a structure of a sensor for detecting a change in a magnetostrictive characteristic of the magnetostrictive film corresponding to a twist generated on a steering shaft in non-contact when a torque acts on the steering shaft from a steering wheel.
In a process for manufacturing the magnetostrictive torque sensor as described above, it is necessary to provide a step of forming a magnetostrictive film on a predetermined surface of a part of the steering shaft, that is, a circumferential surface having a predetermined width in an axial direction in a rotating shaft and adding a magnetic anisotropy to the magnetostrictive film. Referring to a conventional method of adding the magnetic anisotropy to the magnetostrictive film in the manufacture of the magnetostrictive torque sensor, a twist torque is caused to act on the rotating shaft provided with a magnetostrictive material plated portion (magnetostrictive film) by an electrolytic plating treatment, for example, and a stress is applied to the circumferential surface of the rotating shaft to carry out a heat treatment over the rotating shaft by high frequency heating in a stress application state (for example, see JP-A-2004-340744).
When a steering shaft, that is, a rotating shaft provided with a magnetostrictive film constituting a magnetostrictive torque sensor is assembled into an electric power steering apparatus, the rotating shaft is assembled into a housing through a bearing. At that time, it is necessary to prevent the magnetostrictive film formed on the rotating shaft from being damaged in contact with the bearing. For this reason, it may be proposed that a diameter of a region having a predetermined width in an axial direction of the rotating shaft is set to be smaller than a diameter in the vicinity of both ends of the same region and two magnetostrictive films are formed in the region having the predetermined width in the same axial direction.
However, in the case in which the diameter in the region having the predetermined width in the axial direction of the rotating shaft is set to be smaller than the diameter in the vicinity of both ends in the same region and two magnetostrictive films are formed in the region having the predetermined width, the diameters of the rotating shaft in the vicinity of both ends of one of the magnetostrictive films are not equal to each other. For this reason, when high frequency heating is carried out by using an induction heating coil constituted by a one-winding coil portion to perform a heat treatment over one magnetostrictive film, a distance between an outer periphery of the rotating shaft in the vicinity of one of ends of the magnetostrictive film and the induction heating coil is different from a distance between the outer periphery of the rotating shaft in the vicinity of the other end of the magnetostrictive film and the induction heating coil. For this reason, a heating state of the rotating shaft in the vicinity of one of the ends of the magnetostrictive film is different from that of the rotating shaft in the vicinity of the other end. Consequently, there is a problem in that a temperature distribution in the magnetostrictive film is not uniform, it is hard to fabricate a magnetostrictive film having a homogeneous magnetic anisotropy and a detecting sensitivity has a variation.
The contents will be described with reference to FIGS. 12 to 14. FIG. 12 is a longitudinal sectional view showing a main part illustrating a relationship of a disposition in the case in which the magnetostrictive film deposited on the rotating shaft is heated by means of an induction heating coil formed by a one-winding coil portion. In a rotating shaft 100, a diameter d100 of a region 101 having a predetermined width in an axial direction thereof is set to be smaller than a diameter d101 of vicinal portions 102 and 103 on both ends of the region 101. In the rotating shaft 100, a step portion is formed together with the region 101 on both upper and lower ends thereof. Two magnetostrictive films 104 and 105 are formed on the region 101 having the predetermined width in the axial direction. The reference numeral 106 denotes an induction heating coil formed by a one-winding coil portion. As shown in FIG. 12, for example, the diameter d101 of the rotating shaft in the vicinal portion 102 of an end 104A of the magnetostrictive film 104 is not equal to the diameter d100 of the rotating shaft in a vicinal portion 101A for an end 104B. For this reason, a distance D200 between the vicinal portion 102 and the induction heating coil 106 is different from a distance D201 between the vicinal portion 101A and the induction heating coil. When the high frequency heating is carried out by using the induction heating coil 106 formed by the one-winding coil to carry out the heat treatment over the magnetostrictive film 104, therefore, a heating state of the rotating shaft in the vicinal portion 102 of the end 104A of the magnetostrictive film 104 is different from that of the rotating shaft in the vicinal portion 101A of the other end 104B. The foregoing is generated in the same manner when the other magnetostrictive film 105 is heated by the induction heating coil to add a magnetic anisotropy.
FIG. 13 is a graph showing a temperature distribution in the heating of the magnetostrictive film 104. In the graph of FIG. 13, an axis of abscissas indicates a distance (mm) from the end 104A on an upper side of the magnetostrictive film 104 in FIG. 12 and an axis of ordinates indicates a difference from a temperature in a temperature center portion 104C in a percentage (a difference: %) with respect to the temperature of the temperature center portion 104C for each position of the magnetostrictive film 104. A characteristic graph C100 indicates a characteristic of the temperature distribution. As is apparent from FIG. 13, the temperature distribution in the magnetostrictive film 104 is not uniform.
Moreover, FIG. 14 is a graph showing a sensitivity characteristic of a magnetostrictive torque sensor fabricated with the magnetostrictive films 104 and 105 with respect to a heating temperature for the same torque sensor. In the graph of FIG. 14, an axis of abscissas indicates a heating temperature and an axis of ordinates indicates a sensitivity characteristic. A characteristic graph C101 indicates a relationship between the heating temperature and the sensitivity characteristic. As is apparent from FIG. 14, the sensitivity characteristic is changed depending on the heating temperature. For this reason, it is apparent that the sensitivity has a variation when the heating temperature of the magnetostrictive film is not uniform as shown in FIG. 13.